(1) Field of the Invention
This invention relates to a high voltage generator which is useful, for example, for applying high d.c. voltage to a cathode-ray tube of a television receiver, display or the like.
(2) Description of the Prior Art
Heretofore, the high voltage generators which are generally called "flyback transformers" are categorized into two types by the manner of winding the secondary high voltage coil, namely, into a sectional winding type and a coaxial multilayer winding type. The sectional winding type has the high voltage coil divided into a plural number of winding blocks in the axial direction of a high voltage bobbin, while the latter coaxial multilayer winding type has the high voltage coil divided into a plural number of winding layers in the radial direction of the high voltage bobbin. The coaxial multilayer winding type flyback transformer is lately accepted favorably since it can lower the output impedance as a unit in addition to the improvement of the high voltage regulation.
FIGS. 1 to 4 illustrate a prior art high voltage generator using a coaxial multilayer winding type flyback transformer.
Referring first to FIG. 1, the coaxial multilayer winding type flyback transformer which is indicated at 1 has a core 2 which is formed by abuttingly joining a couple of U-shaped members, a low voltage bobbin 3 which is inserted in one leg of the core 2, a primary low voltage coil 4 which is wound in sections on the circumference of the low voltage bobbins 3, a high voltage bobbin 5 which is fitted over the low voltage bobbin 3, a secondary high voltage coil consisting of five winding layers 7A to 7E coaxially wound in multiple layers around the circumference of the high voltage bobbin 5 through an interlay sheet 6, and a high voltage diode 8 consisting of five high voltage diodes 8A to 8E connected in series alternately with the high voltage winding layers 7A to 7E.
Referring to FIG. 2 which shows the general circuit arrangement, the dot "." which is attached to the low voltage coil 4 and the respective high voltage winding layers 7A to 7E indicates a winding end. The high voltage end of the primary coil 4 is connected to a horizontal deflecting circuit 9. The just-mentioned horizontal deflecting circuit 9 is composed of a horizontal output transistor 10 consisting of an NPN type transistor, a damper diode 11, a resonant capacitor 12, a horizontal deflecting coil 13 of a deflecting yoke, and an S-shaped correcting capacitor 14. The collector of the transistor 10 is connected to the high voltage end of the primary coil 4, and the emitter is grounded. The low voltage end of the primary low voltage coil 4 is connected to a flyback power supply 15 which applies a d.c. voltage thereto.
On the other hand, the high voltage coil 7 has its high voltage winding layer 7A at the lowest voltage end connected to an ABL (automatic brightness limitter) or to the ground. The high voltage winding layer 7E at the highest voltage end is connected to the anode terminal 17A of a cathode-ray tube 17 via output high voltage diode 8E and a high voltage cable 16. Indicated at 18 is a fixed resistor on the side of the high voltage output end, and at 19 is a focus volume resistor, which are connected in series between the high voltage output end of the high voltage coil 7 and the ground.
With the high voltage generator of the above-described arrangement, as a basic pulse is applied to the base of the transistor 10 from a horizontal drive circuit (not shown), a collector pulse (flyback pulse) of the collector of the transistor 10 is fed to the low voltage coil 4. As a result, high voltages are induced in the respective winding layers 7A to 7E of the secondary coil 7 according to the number of coil turns, and these high voltages are summed up and rectified through the high voltage diodes 8A and 8E, producing a d.c. high voltage output with a voltage E.sub.H and a current I.sub.H at the high voltage diode 8E as shown in FIG. 3 for supply to the cathode-ray tube 17.
In a case where the flyback transformer 1 has coaxial multiple layer windings as shown in FIG. 1, each with the same number of turns, the potentials between the respective winding layers 7A to 7E become zero a.c.-wise and have the same output waveform. Accordingly, the winding layers 7A to 7E require only an insulation treatment for the d.c. potential difference, and the interlay sheet 6 may be of an extremely small thickness.
On the other hand, a voltage e.sub.H which is expressed by the following equation is produced in the respective winding layers 7A to 7E. EQU e.sub.H =.DELTA.e.sub.H .times.N.sub.H ( 1)
wherein .DELTA.e.sub.H is the voltage which is generated per turn of the winding and N.sub.H is the number of turns of the winding layers 7A to 7E. Therefore, the a.c. through voltage endurance for the distance L (see FIG. 1) between the high voltage end of the winding layer 7A on the lowest voltage side and the primary low voltage coil 4 becomes e.sub.H1 when the a.c. voltage endurance of the primary coil 4 is ignored, namely, the distance L requires a material and a distance which can endure the voltage e.sub.H1. However, in a case where the secondary high voltage coil 7 is divided into five layers, e.sub.H .div.5.4 kV and the distance L takes a small value even if the voltage E.sub.H of the high voltage output is set at 27 kV.
It follows that the finish outer diameter R of the outermost winding layer 7E of the flyback transformer can be minimized, as a result reducing the leakage inductance of the secondary coil 7. Therefore, when the flyback transformer is of the coaxial multilayer winding type as shown in FIG. 1, the high voltage output has the characteristics as indicated at (a) of FIG. 3, with the voltage drop minimized correspondingly to the reduction of the leakage inductance in contrast to the characteristics (b) of the sectional winding type.
In order to cope with another problem of a large variation which occurs to the high voltage output current I.sub.H in the range of 0-200 .mu.A, the fixed resistor 18 and focus volume resistor 19 are inserted in series between the high voltage output terminal and the ground to shunt part of the high voltage output current I.sub.H for improving the high voltage variation and obtaining the characteristics as shown at (c) of FIG. 4. Since the loss will become larger if the resistors 18 and 19 are too small in resistance, current i.sub.H which corresponds to about 10% of the high voltage output current I.sub.H is normally shunted. Energy of 2.6 W is wasted as a loss when current i.sub.H =100 .mu.A is passed through the resistors 18 and 19 with a resistance of 260 M.OMEGA. in total.
Accordingly, assuming that the output impedance of the characteristics (a) of FIG. 3 is Z.sub.01, it is expressed as ##EQU1## On the other hand, assuming that the output impedance of the characteristics (c) of FIG. 4 is Z.sub.02, it is expressed as ##EQU2## showing a conspicuous improvement.
Thus, the above-described prior art contemplated to improve the high voltage regulation by lowering the output impedance through improvement of the characteristics of the flyback transformer 1 as a whole. However, the output impedance of the flyback transformer 1 as a unit has a limitation at about 1.2 M.OMEGA., finding a difficulty in responding to the severe requirements of recent display devices.
Namely, in the characteristics (c) of FIG. 4, the voltage drop .DELTA.E.sub.H due to the variation of the high voltage current I.sub.H in the range of 0-1000 .mu.A is EQU .DELTA.E.sub.H =1000 .mu.A.times.1.2 M.OMEGA.=1.2 kV (4)
Generally, the high voltage regulation Re is expressed as ##EQU3## In the case of the particular example given above, the high voltage regulation Re is ##EQU4## thus corresponding to a variance of 4.8%. Consequently, it is necessary to reduce this variance in order to obtain pictures of high quality.